In the process of testing the Leica M240, I've run into a problem: when pushed at ISOs 200, 400, and 800, the shadows acquire a green cast. In researching the cause of the issue, I’ve determined that the M240 exhibits nonlinear response at ISOs 200, 400, and 800, artificially depressing darker tones. The darker the tone, the greater the depression. The lower the ISO, the greater the depression. The reason that this creates green shadow casts is that the green raw channel is usually the strongest.
Also, I found that adjusting the ISO setting from ISO 200 to ISO 3200 on the M240 gives slightly more gain at each step than is correct. Unlike the green shadows, this is not a problem in normal use.
I obtained raw files exposed to my specifications from another M240 user and determined that his camera exhibited a similar effect; I don’t believe that I have a defective camera – whatever problems it has are the result of its design.
I attempted to model the behavior with simple negative offsets before the amplifiers and before the analog to digital converters, and was unsuccessful. If there are negative offsets, they must vary with ISO setting. I did this work before I discovered the inaccurate ISO setting gains, and may revisit it.
I would be interested in knowing if anyone on this forum has observed the effect, whether they consider it a problem, and what they think causes it.
For myself, I am reluctant to push raw files from the M240 made at the problematic ISOs. Files with no shadow brightening made at these ISOs appear OK to me, because the shadows are dark enough to mask the green cast.
For those interested in the details,
here’s a good place to start. If you have some time on your hands and want to follow my experiments from the beginning,
click here.
Thanks,
Jim
